This invention relates to combination radio/public address systems and particularly to means of switching the same.
It is well known in the art to use the audio amplifier section of a radio receiver to amplify signals other than from the radio tuner portion. U.S. Pat. No. 2,528,636 to Cisin illustrates a switching scheme wherein the audio amplifier of a radio receiver is used to amplify signals transmitted through a telephone system. Signals derived from pickups located in a piano are routed through the audio section of a radio receiver in U.S. Pat. No. 1,994,889 to Hammond. As a further example, U.S. Pat. No. 3,283,076 to Moore illustrates a switching scheme whereby a radio tuner, a phonograph, or microphone outputs are routed through a common amplifying system.
In general, the required switching operation involves modifications to the existing radio circuitry. For example, the lead to the input of the amplifier must be cut with appropriate switching circuitry inserted to accomplish the multifunction operation. The switches, either manual or relays, are expensive and take up considerable space, which, in mobile applications, is a considerable disadvantage. Furthermore, many such switching schemes are subject to radio feedthrough. Radio feedthrough occurs when the amplifier is being used for other than radio signal amplification yet the radio pickup can be detected in the background. This of course is annoying and distracting.
A public address system may use the radio receiver's audio amplifier with a microphone input. A good microphone for public address applications is the Shure Model 414C which includes a build-in preamp and a pair of ganged leaf switches in one self contained unit. As with most microphone units, the 414C is desiged to be operated into a fixed load for maximum performance.